The invention relates to a layer system for the rotor/stator seal of a turbomachine which is applied to a metallic component and can be run in with respect to a further component which can move relative to the metallic component, and to a turbomachine, in particular a gas turbine.
To keep undesirable gap losses which affect the efficiency of gas turbine engines at a low level, it is necessary to keep the gap between the rotor blades, which rotate at high speed, and the housing associated with the stator which surrounds the rotor small for as long as possible, i.e. to achieve the optimum possible sealing action.
It is known that the size of this gap is not constant but rather is subject to changes during the different operating phases of the gas turbine engine. For example, inter alia, the rotor blades, under high operating loads resulting from the thermal stresses and the centrifugal force, are subject to extension in the radial direction, while the housing is only subject to thermal expansion. These different thermal reaction characteristics lead to the observed changes in the gap size.
To ensure that the different thermal reaction characteristics of rotor blades and the housing and the requirement for a small gap size between the blade tips and the housing are taken into account, it is known to allow initial stripping or running-in to take place between the tips of the rotor blades and the surrounding housing. To prevent this from causing damages to the tips of the rotor blades and to the surrounding housing, an abrasive coating has often been applied to the tips of the rotor blades, while the surrounding housing is lined with a suitable run-in coating.
During the initial stripping or running-in of the blade tip at the run-in coating which has been introduced into the surrounding housing, the abrasive particles which have been applied to the blade tip cut or rub into the run-in coating, thereby abrading it.
To keep the wear to the blade tips as low as possible, it is also known to use relatively soft run-in coatings, i.e. run-in coatings with a low resistance to abrasion. Moreover, run-in coatings with a low resistance to abrasion provide a good thermal insulation and, on account of their good run-in properties, allow run-in depths of a few millimetres without damages to the blades.
A run-in coating of this type is described and illustrated in U.S. Pat. No. 5,434,210 and U.S. Pat. No. 4,936,745.
However, one of the disadvantages of the soft run-in coatings is that they are susceptible to erosion and have a poor ability to withstand temperature changes, leading to a restricted service life.
EP 0 765 951 B1 discloses a run-in coating system with three ceramic layers which are segmented and are made from hollow sphere powder. A drawback of this system is that the ceramic (base) layer which adjoins the bonding layer is relatively unresistant to erosion, on account of the hollow sphere powder. Moreover, on account of the interlayer, which is of graded composition, the system is expensive and complex to produce.